1. Field of the Invention
The present invention relates to a process and an apparatus or cleaning ammonia-containing exhaust gas. More particularly, the present invention pertains to a process and an apparatus for cleaning ammonia-containing exhaust gas which is exhausted from a semiconductor manufacturing process or a chemical treatment process.
2. Description of the Related Arts
Ammonia has widely been used in recent years in each of manufacturing processes with continuous development of a semiconductor manufacturing industry, an optoelectronics industry, a precision machinery and instrument manufacturing industry, a cemented carbide material industry, an ornament manufacturing industry and the like. Among the foregoing industries, a large amount of ammonia is used in the manufacturing process of a nitride film of a compound semiconductor.
Although ammonia is an indispensable substance in the manufacturing process of a compound semiconductory, it is highly toxic, and thus has a threshold limit value in time weighted average (TLV-TWA) of 25 ppm. Therefore, in the case where an ammonia-containing exhaust gas is exhausted in the atmosphere, it exerts evil influence upon human bodies and environment. Accordingly, it is necessary to clean, prior to exhaust in the atmosphere, an ammonia-containing exhaust gas which is exhausted after ammonia is used in a semiconductor manufacturing process or the like.
In addition, a large amount of ammonia is used in some chemical industries. In this case, an ammonia-containing exhaust gas is exhausted as the case may be, and needs to be cleaned prior to exhaust into the atmosphere.
There have heretofore been known as a process for cleaning ammonia-containing exhaust gas, 1 a process in which an ammonia-containing exhaust gas is brought into contact with an acidic aqueous solution of sulfuric acid or the like so as to clean the gas by capturing the ammonia contained therein as a salt such as ammonium sulfate; 2 a process in which an ammonia-containing exhaust gas is introduced in a combustion furnace so as to clean the gas by converting the ammonia into water and nitrogen; 3 a process in which an ammonia-containing exhaust gas is brought into contact with a dry system ammonia harm-removing agent to clean the gas; 4 a process in which an ammonia-containing exhaust gas is brought into contact with an ammonia decomposition catalyst under heating so as to clean the gas by converting the ammonia into nitrogen and hydrogen ; 5 a process for cleaning an ammonia-containing exhaust gas by the combination of the ammonia decomposition catalyst and the dry system ammonia harm-removing agent; and the like process.
However, the above-mentioned processes involves such problems as described hereunder.
The cleaning process 1 of bringing into contact with an acidic aqueous solution of sulfuric acid or the like suffers from the defect of by-producing a large amount of an ammonium salt at the time of cleaning treatment. The cleaning process 2 by combustion in a combustion furnace suffers from not only the disadvantage that the process is possibly accompanied with incomplete combustion, abnormal combustion or the danger of explosion because of the variations in the gas flow rate, combustible components and the like, since the exhaust gas is not exhausted under stationary state in many cases, but also the drawback that harmful nitrogen oxides are generated by the combustion treatment. The cleaning process 3 by bringing the gas into contact with a dry system ammonia harm-removing agent suffers from the shortcomings that in the case of treating a large amount of ammonia contained in the gas, a large amount of the harm-removing agent is needed, whereby a great expense is incurred and further, a large sized treatment unit is required in cleaning treatment. The cleaning process 4 by bringing the gas into contact with an ammonia decomposition catalyst under heating suffers from the disadvantage that a part of ammonia remains undecomposed in a concentration of several tens to several hundreds ppm in the cleaned exhaust gas, since the ammonia decomposition ratio is specified by the chemical equilibrium among the reactants. In addition, the cleaning process 5 by the combination of the decomposition catalyst and the harm-removing agent suffers from the drawback that a large amount of the harm-removing agent is needed owing to a high ammonia concentration of several tens to several hundreds ppm in the treated exhaust gas.
Under such circumstances, it has been eagerly desired to develop and provide an efficient process and a compact apparatus which exhibit high cleaning performance, facilitate the treatment after the cleaning, minimize ammonia effluence in the exhaust gas after cleaning and are capable of treating an ammonia-containing gas at a low cost.